1. Technical Field
The present invention relates to modems in general, and in particular to non-echo cancelling voice band modems. Still more particularly, the present invention relates to a method and apparatus for detecting subscriber alerting signal tones and customer premises equipment alerting signal tones in a non-echo canceling voice band modem.
2. Description of Related Art
If a telephone user is a subscriber to the call-waiting service, the telephone user will receive an audible tone of certain frequency and cadence on a telephone to inform the telephone user of an incoming call while the user is already attending a call on the telephone. If the telephone user is also a subscriber to the caller identification on call waiting (CIDCW) service, the telephone user will receive a Caller ID of the incoming call in conjunction with the audible tone to inform the telephone user the identity of the caller of the incoming call. The audible tone associated with the call-waiting service is implemented by subscriber alerting signals (SASs) or commonly known as call-waiting signals. Each Caller ID associated with the CIDCW service is preceded by a customer premises equipment alerting signal (CAS).
SAS signals are different for different countries. In the United States of America, SAS signals are single tone signals having a frequency of 440 Hz. On the other hand, CAS signals are dual-tone signals having frequencies 2,130 Hz and 2,750 Hz. Both SAS and CAS signals need to be detected in order to take advantage of the call-waiting service and the CIDCW service, respectively.
For non-echo canceling modems, the detections of SAS and CAS signal tones have to be performed in the presence of echoes of transmitted signals. Such operating environment deteriorates modem performance, especially when the modem has to comply with rigorous standards such as TIA/EIA-777. Consequently, it would be desirable to provide an improved method and apparatus for detecting SAS and CAS signal tones in non-echo canceling modems.